How do you convert a number of jpg files to JPEG files all at once?

There is basically no difference between jpg and jpeg. They mean the same thing, and are exactly the same type of file. It's just a different name (extension) for the same thing.

However if you want to convert these type of images in bulk, ImageBadger Image Converter will help and save your time.

Source: standaloneinstaller.com/download-imagebadger-image-converter